Metal halide lamps began with the addition of the halides of various light-emitting metals to the high pressure mercury vapor lamp in order to modify its color and raise its operating efficacy as proposed by U.S. Pat. No. 3,234,421 - Reiling, issued in 1966. Since then metal halide lamps have been widely used for general illumination of commercial and industrial places and in outdoor lighting. Their construction and mode of operation are described at pages 8-34 of IES Lighting Handbook, 5th Edition, 1972, published by the Illuminating Engineering Society.
The metal halide lamp generally operates with a substantially fully vaporized charge of mercury and an unvaporized excess consisting mostly of metal iodides in liquid form. One filling which has been favored comprises the iodides of sodium, scandium and thorium. The operating conditions together with the geometrical design of the lamp envelope must provide sufficiently high temperatures, particularly in the ends, to vaporize a substantial quantity of the iodides, especially of the NaI. In general, this requires minimum temperatures under operating conditions of the order of 700.degree. C.
In U.S. Pat. No. 4,161,672 - Cap et al, July 1979, miniature metal halide arc tubes are disclosed which utilize thin-walled fused silica envelopes with small end seals and achieve high efficacy in discharge volumes of 1 cubic centimeter or less. Those miniature arc tubes are particularly useful as the principal light source in lighting units designed for functional similarity to common incandescent lamps. For such applications a low color temperature matching that of the incandescent lamp which has a color temperature of about 2900 K. is particularly desirable. The color temperature of current metal halide lamps containing a dose of NaI/ScI.sub.3 /ThI.sub.4 is typically around 4200 K. or above for a clear lamp. By applying a phosphor favoring the low side of the spectrum to the outer envelope, the effective color temperature may be lowered to 3800 K. but this reduces efficiency and still falls short of the objective.
It is possible to lower the color temperature of NaI-containing lamps by increasing the relative sodium concentration in the arc. This may be achieved by changing physical construction parameters such as arc tube size, length to diameter ratios, and electrode lengths. The effect of the physical construction changes must be to increase the temperature of the halide pool thereby increasing the sodium pressure to yield a lower color temperature lamp. As a consequence of the reactive nature of the metal halides used, increasing the average wall temperature increases the rate of deleterious chemical reaction processes which can result in poor maintenance and short life. These unwanted effects are aggravated by small envelope volume in miniature lamps.
Another mechanism which may be used for lowering color temperature in NaI-containing lamps is a mercury density in the discharge space high enough to broaden the sodium D line (589 nm) into the red region. By using this mechanism with miniature metal halide lamps we have achieved color temperatures as low as 3500 K. but this is still short of the 2900 K. objective.
In the copending application of John E. Spencer and Ashok K. Bhattacharya, Ser. No. 93,899, filed Nov. 13, 1979, Metal Halide Lamp Containing ThI.sub.4 With Added Elemental Cadmium or Zinc and now U.S. Pat. No. 4,360,756, improved maintenance is sought in a lamp using a thorium-tungsten cathode. Such an electrode is formed by operating a tungsten cathode, generally a tungsten rod having a tungsten wire coiled around it in a thorium iodide-containing atmosphere. Under proper conditions the rod acquires a thorium spot on its distal end from the ThI.sub.4 dosed into the lamp. This thorium then serves as a good electron emitter which is continually renewed by a transport cycle involving the halogen present which returns to the cathode any thorium lost by any process. The thorium-tungsten cathode and its method of operation are described in Electric Discharge Lamps by John F. Waymouth, M.I.T. Press, 1971, Chapter 9. Spencer and Bhattacharya found that the proper operation of the thorium transport cycle is suppressed when excess or free iodine is present in the lamp atmosphere during operation. They teach as remedy adding a getter in the form of a metal whose free energy of formation as an iodide compound must be more negative than that of HgI.sub.2 but less negative than that of the ThI.sub.4. They propose as getters the metals Cd, Zn, Cu, Ag, In, Pb, Cd, Zn, Mn, Sn and Tl.